Single spin asymmetry for the Drell-Yan process

We calculated the single spin asymmetries for the reaction P + P(↑) → ll + X in the framework of twist-3 QCD for HERA energies. The necessary imaginary phase is produced by the on-shell contribution of the quark propagator, while the long distance part is analogous to that providing the direct photon asymmetry calculated by J. Qiu and G. Sterman [Nucl. Phys. B 378 (1992) 52]. The asymmetry turns out to be generally of the order percent.